Theory

A perfect fifth in meantone, and in common practice theory, consists of four diatonic semitones and three chromatic semitones. In 12edo, and hence in most discussions these days, that is simplified to seven semitones. On the other hand, in just intonation it consists of four just diatonic semitones of 16/15, three just chromatic semitones of 25/24, and two Didymus commas of 81/80, and is the just perfect fifth of 3/2.